Radiotelephone systems are generally well known in the art. A particular type of radiotelephone system is a cellular radiotelephone system. Cellular radiotelephone systems generally include a switch controller coupled to a public switched telephone network (PSTN) and a plurality of base stations. Each of the plurality of base stations generally defines a geographic region proximate to the base station to produce coverage areas. One or more mobile stations communicate with a base station that facilitates a call between the mobile station and the public switched telephone network. A detailed description of a cellular radiotelephone system is described in the book "Mobile Cellular Communications Systems" by Dr. William C. Y. Lee, 1989.
A handoff in a cellular radiotelephone system is defined as a transition between the mobile station communicating with a first base station supporting a first coverage area and the mobile station communicating with a second base station supporting a second coverage area. The handoff may be either base controlled, mobile controlled, or base and mobile controlled. A mobile assisted handoff (MAHO) is defined as a base controlled handoff that uses information provided by the base station as well as information provided by the mobile station.
Handoffs are further characterized as being hard hand offs or soft handoffs. During a hard handoff communication between the mobile station and the first base station is terminated prior to the initiation of communication between the mobile station and the second base station. During a soft handoff, communication between the mobile station and the second base station is initiated prior to the termination of communication between the mobile station and the first base station. Cellular radiotelephone communication systems employing soft handoffs typically use the same radio frequency (RF) channel for communication between the mobile station and the first base station or the second base station.
A problem in cellular radiotelephone communication systems is that the communication between the mobile station and the switch controller is terminated when a needed handoff is unsuccessful. This termination is otherwise known as a dropped call. Although mobile assisted handoffs and soft handoffs have reduced the frequency of dropped calls, a condition in cellular radiotelephone communication systems exists which still causes calls to be dropped. The condition is when the mobile station is in an active call in the first coverage area with the first base station and substantially shadowed from transmissions from the second base station in the second coverage area. When the mobile station suddenly transitions from the first coverage area to the second coverage area, the mobile station experiences a substantial increase in the signal transmissions from the second base station relative to the signal transmissions from the first base station. If the substantial increase in signal transmissions from the second base station occurs over a relatively short time duration, the signal transmissions from the second base station interfere with the communication between the mobile station and the first base station thereby preventing handoff messaging and resulting in the dropped call.
Prior art provides a cellular radiotelephone system with dropped call protection in U.S. Pat. No. 4,811,380. This reference teaches an improved cellular telephone communication system having operational steps which prevent a call from being dropped due to a radiotelephone not receiving a handoff instruction from its host base site. The system includes a switch controller for determining that the radiotelephone requires a handoff from a first base site coverage area to a second base site coverage area and for communicating a handoff message to the associated first and second base site equipment. The first base site equipment then transmits the handoff message to the radiotelephone. If the radiotelephone does not receive the message, it determines that the call has been lost, seizes a signaling channel from the second base site, and requests, via the second base site a call reconnection by transmitting a special message. The second base site then informs the radiotelephone of the handoff instruction and the handoff is completed with a successful reconnection of the call.
A problem with the technique described in U.S. Pat. No. 4,811,380 is that the mobile station only takes action to recover a call after the absence of a signal transmission from the first base station over a predetermined period of time. The prior art mobile station does not predict the changing RF conditions that are setting up a self-interference condition in the radiotelephone system that requires a hand-off. Since the prior art mobile station does not predict the changing RF conditions, either the call is dropped before the handoff messaging can be successfully completed or the time delay for the recovery of the dropped call is unacceptable to a user of the mobile station.
Accordingly, there is a need for a method, performed by the mobile station, for preventing the active call from being dropped when the mobile station suddenly transitions from the first coverage area to the second coverage area of the radiotelephone system that overcomes the disadvantages of a dropped call or the time delay resulting from the recovery of the dropped call.